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Activation of Carbon-Hydrogen Bonds via 1,2-Addition
across M-X (X ) OH or NH2) Bonds of d6 Transition Metals
as a Potential Key Step in Hydrocarbon Functionalization:
A Computational Study
Thomas R. Cundari,*,† Thomas V. Grimes,† and T. Brent Gunnoe*,‡
Contribution from the Department of Chemistry and Center for AdVanced Scientific
Computing and Modeling (CASCaM), UniVersity of North Texas, P.O. Box 305070, Denton,
Texas 76203-5070, and Department of Chemistry, North Carolina State UniVersity,
Raleigh, North Carolina 27695-8204
Received June 6, 2007; E-mail: [email protected]
Abstract: Recent reports of 1,2-addition of C-H bonds across Ru-X (X ) amido, hydroxo) bonds of TpRu(PMe3)X fragments {Tp ) hydridotris(pyrazolyl)borate} suggest opportunities for the development of new
catalytic cycles for hydrocarbon functionalization. In order to enhance understanding of these transformations,
computational examinations of the efficacy of model d6 transition metal complexes of the form [(Tab)M(PH3)2X]q (Tab ) tris-azo-borate; X ) OH, NH2; q ) -1 to +2; M ) TcI, ReI, RuII, CoIII, IrIII, NiIV, PtIV) for
the activation of benzene C-H bonds, as well as the potential for their incorporation into catalytic
functionalization cycles, are presented. For the benzene C-H activation reaction steps, kite-shaped transition
states were located and found to have relatively little metal-hydrogen interaction. The C-H activation
process is best described as a metal-mediated proton transfer in which the metal center and ligand X
function as an activating electrophile and intramolecular base, respectively. While the metal plays a primary
role in controlling the kinetics and thermodynamics of the reaction coordinate for C-H activation/
functionalization, the ligand X also influences the energetics. On the basis of three thermodynamic criteria
characterizing salient energetic aspects of the proposed catalytic cycle and the detailed computational
studies reported herein, late transition metal complexes (e.g., Pt, Co, etc.) in the d6 electron configuration
{especially the TabCo(PH3)2(OH)+ complex and related Co(III) systems} are predicted to be the most
promising for further catalyst investigation.
1. Introduction
The development of catalysts for the functionalization of
carbon-hydrogen bonds is an important pursuit that could
impact both commodity and fine chemical markets. Transition
metal systems that initiate stoichiometric metal-mediated activation of carbon-hydrogen bonds are known,1 and many of these
systems function at ambient conditions. The most commonly
invoked mechanisms for the C-H bond cleavage step include
oxidative addition (OA), σ-bond metathesis (SBM), and electrophilic substitution (ES). Despite the success of metal-mediated
C-H activation, the incorporation of stoichiometric C-H
activations into catalytic cycles remains relatively rare,2 and this
is especially true for the functionalization of aliphatic hydrocarbons. Noteworthy examples of catalytic conversions include
†
‡
University of North Texas.
North Carolina State University.
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13172
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J. AM. CHEM. SOC. 2007, 129, 13172-13182
alkane dehydrogenation,3-5 alkane metathesis,6 alkane silylation,7 alkane borylation,8 as well as hydrocarbon functionalization by Shilov-type metal electrophiles.9 In an interesting tandem
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10.1021/ja074125g CCC: $37.00 © 2007 American Chemical Society
Key Step in Hydrocarbon Functionalization
Scheme 1. Oxidative Addition (OA) and σ-Bond Metathesis (SBM)
Mechanisms for C-H Bond Activation; q Denotes the Formal
Oxidation State of the Metal
In the typical OA mechanism, both the carbon and hydrogen
of the C-H bond that is cleaved are transferred to a low-valent
transition metal center via a three-centered transition state,
Scheme 1.11 In contrast to the three-center transition state
associated with OA, SBM is a concerted reaction involving four
atomic centers including the metal center, the ligand that receives
the transferred proton, and the C and H atoms of the bond being
activated, Scheme 1. The SBM pathway leaves the formal
oxidation state of the metal center unchanged. Cundari has
contrasted OA and SBM pathways of carbon-hydrogen bond
activation in terms of metal complex to substrate electron
donation and backdonation.12 Both OA and SBM are characterized by an “electrophilic” phase with dominant substrate to metal
donation early (i.e., before the transition state) in the reaction
coordinate. A “nucleophilic” phase dominated by metal complex
to substrate backdonation follows and serves to delineate the
mechanisms.12 In typical (i.e., monometallic complex) OA
pathways, the metal acts as both electrophile and nucleophile,
and thus both ends of the C-H bond being activated end up on
the metal. For SBM systems, the donor orbital on the metal
complex is a metal-ligand frontier orbital polarized toward the
more electronegative ligand. Periana and Goddard et al. have
proposed an oxidative-hydrogen migration (OHM) mechanism
for C-H activation by Ir(III) complexes as a variant of OA
and SBM.13 The OHM transition state is in some respects
intermediate between OA and SBM14 transition states (see
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ARTICLES
Scheme 2. Possible Routes for Catalytic C-H Functionalization
That Involve Net 1,2-Addition of C-H Bonds across M-X(R)
Bonds (X ) O or NR)
below) and is distinguished from the latter by more significant
interaction between the metal and the hydrogen of the carbonhydrogen bond being activated. The distinction between OA,
OHM, and SBM is reminiscent of many debates in chemical
bonding; while technical distinctions exist and demarcation into
categories can be useful, real mechanisms may lie on a spectrum
defined by these three classifications.
In addition to OA, SBM, and ES pathways for C-H
activation, the net 1,2-addition of C-H bonds across M-X (X
) heteroatomic ligand such as amido, alkoxo, imido, oxo, etc.)
bonds also holds promise as a step in overall catalytic C-H
functionalization.15 Only a few examples of net 1,2-addition of
C-H bonds across M-X bonds have been reported. For
example, the Wolczanski16 and Bergman17 groups have studied
the 1,2-addition of C-H bonds, including that of methane for
the former group, across d0 metal-imido (M ) NR) bonds of
early transition metals such as Ti and Zr. It has been established
that the reaction is an overall [2σ + 2π] addition and that the
transition state has a four-centered arrangement preceded by
an alkane or arene adduct. The resulting alkyl/aryl product is
only a single C-N reductive elimination step away from
substrate functionalization to produce amine. However, reductive
elimination is difficult for electropositive early transition metal
complexes. In contrast, precedent for C-N and C-O reductive
elimination from late transition metals is extensive.18 Thus,
extension of the net 1,2-addition of C-H bonds to late transition
metal systems might ultimately be incorporated into catalytic
cycles for C-H functionalization.
(15) (a) Tenn, W. J., III; Young, K. J. H.; Bhalla, G.; Oxgaard, J.; Goddard, W.
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(18) The proclivity of late transition metals for C-O and C-N has been
profitably exploited in, for example, the well-known Hartwig-Buchwald
etheration and amination reactions. (a) Reductive: Stuermer, R. In Organic
Synthesis Highlights V; Schmalz,; H.-G., Wirth, T., Eds.; Wiley-VCH: New
York, 2003; p 22. (b) Muci, A. R.; Buchwald, S. L. Top. Curr. Chem.
2002, 219, 131. (c) Hartwig, J. F. In ComprehensiVe Coordination
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J. AM. CHEM. SOC.
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Cundari et al.
ARTICLES
Scheme 2 shows two pathways in which 1,2-addition of C-H
bonds across M-X (X ) O or NR) or M-XR bonds could be
incorporated into a catalytic cycle. Route A involves the 1,2addition of a C-H bond across a metal-heteroatom (MdX)
bond, which is followed by reductive elimination of functionalized product (R-X-H). Regeneration of the active species
occurs via an atom- or group-transfer reagent. This route is
perhaps similar in some respects to metal-catalyzed insertion
of carbenes into C-H bonds.19 Route B involves the initial
functionalization of a metal-alkyl moiety via formal insertion
of an oxygen atom or nitrene fragment into a metal-alkyl or
metal-aryl bond. Hydrogen transfer from a hydrocarbon
substrate follows to make the functionalized product and
regenerate the active metal-alkyl species. Mayer and Brown
have reported high-valent Re-oxo complexes that undergo oxo
insertion into Re-Ph bonds under photolytic and thermal
conditions.20 In addition, Periana et al. have recently reported
remarkably facile conversions of the Re-Me bond of methylrheniumtrioxo to a methoxo ligand upon treatment with various
oxidants,21 with preliminary mechanistic studies suggesting that
the inserted oxygen atom is not derived from a RedO ligand.
Recently, our groups have become interested in extending
1,2-addition of C-H bonds across M-X bonds to late transition
metals in relatively low oxidation states. While high valent late(r) transition metal-oxo and hydroxo/alkoxo complexes are
prevalent in the functionalization of C-H bonds, these systems
typically activate C-H bonds through net radical hydrogenatom abstraction routes for which the metal center does not
directly interact with the C-H bond, but rather, the metal serves
(19) (a) Braga, A. A. C.; Maseras, F.; Urbano, J.; Caballero, A.; Mar DiazRequejo, M.; Perez, P. J. Organometallics 2006, 25, 5292. (b) de Fremont,
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Scheme 3. Three Pathways to Cleave C-H Bonds by Transition
Metal Systems with Formally Anionic Heteroatomic Ligands
as an oxidizing “electron reservoir.”22-24 Our working hypothesis is that lower oxidation states increase the propensity toward
even-electron processes such as intermolecular heterolytic C-H
bond cleavage and intramolecular 1,2-addition reactions (Scheme
3).25 For example, it has been demonstrated that coordinatively
and electronically saturated octahedral Ru(II) and Fe(II) amido
complexes can deprotonate (i.e., an even-electron transformation) relatively acidic C-H bonds including those of phenylacetylene, 1,4-cyclohexadiene, and triphenylmethane. In one
prominent study, Bergman et al. investigated the reactivity of
trans-(dmpe)2Ru(H)(NH2) toward acidic C-H bonds.26 Even
very weakly acidic compounds such as toluene react, though
the reaction is endothermic.26 The corresponding Fe(II) analogue
shows similar reactivity, although it is less basic than the
ruthenium complex.27 TpRuL2X (X ) NHR, OR; Tp ) hydridotris(pyrazolyl)borate) and (PCP)Ru(L)(NHR) (PCP ) bisphosphine “pincer” ligand) systems exhibit similar reactivity.28-30
It is important to emphasize that these transformations likely
involve intermolecular C-H bond cleavage without direct
metal-CH interaction (Scheme 3).
As we have previously suggested for reactions of (PCP)Ru(CO)NH2,29 the coordination of C-H bonds to similar systems26,27 (i.e., amido, hydroxo, or related complexes in low
oxidation states) could activate them toward a net intramolecular
deprotonation by the highly basic non-dative ligand “X”
(Scheme 3). We have reported evidence for such reactions with
TpRu(PMe3)X (X ) OH, OPh, or NHPh) systems,15b,c and
Periana et al. have directly observed related reactions with an
Ir(III)-methoxo complex.15a,31 To our knowledge, these reports
are the only examples of 1,2-addition of C-H bonds across
M-X (X ) OR, NHR, O, or NR) bonds for late transition
metals in low oxidation states. In related chemistry, Macgregor
et al. have reported the possible involvement of an Ir-acetate
(24) Au, S. M.; Huang, J. S.; Yu, W. Y.; Fung, W. H.; Che, C. M. J. Am.
Chem. Soc. 1999, 121, 9120.
(25) Gunnoe, T. B. Eur. J. Inorg. Chem. 2007, 1185.
(26) (a) Fulton, J. R.; Bouwkamp, M. W.; Bergman, R. G. J. Am. Chem. Soc.
2000, 122, 8799. (b) Fulton, J. R.; Sklenak, S.; Bouwkamp, M. W.;
Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 4722. (c) Holland, A. W.;
Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 14684.
(27) Fox, D. J.; Bergman, R. G. Organometallics 2004, 23, 1656.
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Inorg. Chem. 2002, 41, 3042. (c) Conner, D.; Jayaprakash, K. N.; Wells,
M. B.; Manzer, S.; Gunnoe, T. B.; Boyle, P. D. Inorg. Chem. 2003, 42,
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Key Step in Hydrocarbon Functionalization
ARTICLES
Scheme 4. Proposed Reaction Pathway for the 1,2-Addition of
Benzene across M-X Bonds Studied by DFT Calculations
Figure 1. Calculated benzene adduct [(Tab)Pt(PH3)(benzene)OH]2+. All
hydrogen atoms are omitted for clarity, except the hydroxo hydrogen and
the hydrogen attached to the ligated carbon of benzene.
3. Results and Discussion
ligand in C-H bond activation,32 and 1,2-addition of a C-H
bond across a Pt-Cl bond has been implicated in Shilov-type
chemistry.9 Thus, little is known about the Various factors that
impact the energetics of the 1,2-additions. Herein, we report a
comprehensive computational study that addresses the influence
of the identity of the nondative ligand (OH vs NH2) and metal
[Tc(I), Re(I), Ru(II), Co(III), Ir(III), Ni(IV), and Pt(IV)] on the
kinetics and thermodynamics of benzene C-H bond scission
by 1,2-addition. While the present studies are specifically
focused on [(Tab)M(PH3)X]n systems, the results provide
guidance for the design of systems that are more active for C-H
bond activation, particularly within a catalytic cycle for
hydrocarbon functionalization.
2. Computational Methods
All geometries were optimized within the Jaguar33 program with
density functional theory (DFT) using the B3LYP functional.34 The
Stevens effective core potential (ECP) and valence basis set was used,35
with a d-polarization function on heavy main group elements (CSDZ*
in Jaguar). Each structure (with exceptions as indicated) was confirmed
as a minimum using an energy Hessian calculation. The tris-pyrazolyl
borate (Tp) ligand was replaced with tris-azo borate (Tab), which was
shown in previous work36 to behave similarly in electronic, energetic,
and steric impact to the full Tp ligand. Bader’s Atoms In Molecules
(AIM37) analysis was performed in Gaussian 0338 using B3LYP/3-21G*
(one of the largest all-electron basis sets for Ru available through
EMSL39) with the B3LYP/CSDZ* geometries as optimized in Jaguar.
An all-electron basis set is necessary to use the AIM analysis. Natural
population analysis (NPA)40 was performed in Jaguar to calculate atomic
charges.
(32) Davies, D. L.; Donald, S. M. A.; Macgregor, S. A. J. Am. Chem. Soc.
2005, 127, 13754; Davies, D. L.; Donald, S. M. A.; Al-Duaij, O.;
Macgregor, S. A.; Pölleth, M. J. Am. Chem. Soc. 2006, 128, 4210.
(33) Jaguar 5.5, Schrödinger, L. L. C. Portland, OR, 1991-2003.
(34) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(35) (a) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102,
939. (b) Stevens, W. J.; Basch, H.; Krauss, M. J. Chem. Phys. 1984, 81,
6026. (c) Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem.
1992, 70, 612.
(36) Bergman, R. G.; Cundari, T. R.; Gillespie, A. M.; Gunnoe, T. B.; Harman,
W. D.; Klinkman, T. R.; Temple, M. D.; White, D. P. Organometallics
2003, 22, 2331.
(37) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford University
Press: Oxford, 1990.
(38) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford
CT, 2004.
(39) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi,
V.; Chase, J.; Li, J.; Windus, T. L. J. Chem. Inf. Model 2007, 47, 10451052.
(40) Weinhold, F.; Landis, C. R. Chem. Ed.: Res. Pract. Eur. 2001, 2, 91.
3.1. Structure and Bonding Considerations. We have
previously reported evidence that complexes of the type TpRu(PMe3)2X (X ) OH or NHPh) initiate the 1,2-addition of
aromatic C-H bonds across the Ru-X bond.15b,c Experimental
and computational studies suggest that the transformations
proceed via initial dissociation of PMe3 and coordination of
benzene to form TpRu(PMe3)(benzene)X complexes, followed
by C-H activation via 1,2-addition. Computational studies
incorporated the smaller ligand models Tab (tris(azo)borate) and
PH3 in place of Tp and PMe3, respectively. The proposed
reaction pathway is depicted in Scheme 4.
We have extended computational studies of the overall
benzene C-H activation shown in Scheme 4 to a series of
octahedral d6 complexes in which the identity of the metal and
ligand X are varied: [(Tab)M(PH3)2X]q (X ) OH or NH2; M
) Tc or Re, q ) -1; M ) Ru, q ) 0; M ) Co or Ir, q ) +1;
M ) Ni or Pt, q ) +2). Since Tab is a tridentate, six-electron
donor ligand, complexes A, C, E, and F are formally 18electron, six-coordinate, and octahedral (Scheme 4). Species B,
[(Tab)M(PH3)X]q, is formally a 16-electron, five-coordinate
complex. However, the π-donation capability of X can render
the active species (B) closer to 18-electron, as judged by the
planar coordination mode at the nitrogen (when X ) NH2) of
the amido ligand in these complexes (see below).41 All
coordination geometries of optimized minima for complexes
A, E, and F are as expected, and the DFT calculations present
no surprises in this regard. However, structures C and Dq, the
benzene adduct and the C-H activation transition state,
respectively, vary among the complexes. More detailed analysis
of these systems is given below.
3.1.1. Benzene Adduct Geometries. For the overall C-H
activation of benzene, the arene adducts (C in Scheme 4) can
not only impact the rate of the overall reaction but also enhance
the Arrhenius prefactor. The benzene adducts in the proposed
pathway for C-H activation serve to align the substrate arene
with the activating ligand X, preparing the complex for carbonhydrogen bond activation. The pertinent complexes shall be
designated briefly as [M-X]q to indicate the specific metal (M),
activating ligand (X) and overall charge of the complex (q).
Benzene adducts were found for [Ir-OH]+, [Ni-OH]2+, [PtOH]2+, [Ni-NH2]2+, and [Pt-NH2]2+ complexes with a
representative structure shown in Figure 1 for [(Tab)Pt(PH3)(benzene)OH]2+. In contrast, related benzene adducts were not
located for [Tc-OH]-, [Re-OH]-, [Ru-OH], [Co-OH]+,
[Tc-NH2]-, [Re-NH2]-, [Ru-NH2], [Co-NH2]+ or [Ir(41) For all metals, in A the amido ligand is pyramidal, suggesting σ-only
donation. In the active species B, however, the amido ligand is planar,
allowing two-electron π-donation to satisfy the 18-electron rule.
J. AM. CHEM. SOC.
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ARTICLES
Table 1. Representative Geometric Data for Calculated Benzene
Adducts [(Tab)M(PH3)(benzene)X]q a
(Tab)M(PH3)(benzene)OH
metal
MC1 (Å)
MC2 (Å)
Bz C−C (Å)
Bz C−H oop (deg)
Tc
Re
Ru
Co
Ir
Ni
Pt
4.41
4.41
3.95
3.07
2.59
2.42
2.40
4.37
4.44
4.23
3.30
3.02
3.03
3.09
1.40
1.40
1.40
1.40
1.42
1.43
1.44
0.3
0.5
1.5
4.5
14.8
17.7
27.6
(Tab)M(PH3)(benzene)NH2
metal
MC1 (Å)
MC2 (Å)
Bz C−C (Å)
Bz C−H oop (deg)
Tc
Re
Ru
Co
Ir
Nib
Pt
4.26
4.56
5.14
4.59
4.69
2.61
2.49
4.39
4.95
5.18
4.76
4.97
3.13
3.13
1.40
1.40
1.40
1.40
1.40
1.42
1.43
0.1
0.0
0.5
3.0
2.3
14.1
23.6
a C and C are the two benzene carbon atoms closest to the metal center.
1
2
‘Bz C-H oop’ is the out-of-plane bending angle of the activated H from
the plane of the benzene ring. b A benzene adduct was located for NiNH2 using a different SCF convergence algorithm (iacscf ) 1 in Jaguar).
At the optimized geometry, the usual SCF algorithm reports a small
imaginary mode of 13i cm-1.
NH2]+. With the interaction of the benzene adducts occurring
to a single carbon, the calculated adducts structurally resemble
intermediates expected in electrophilic aromatic substitution
(EAS). In the classical EAS mechanism, an electrophile attacks
a carbon of the arene ring to form an arenium intermediate,
and subsequent intermolecular deprotonation restores aromaticity
to yield the substituted aromatic product. In the present EAS
analogy, the base for metal-mediated C-H activation is the
ligand X, which serves as an intramolecular base.42 Since this
mechanism is expected to be restricted to aromatic substrates,
the present mechanism may more properly be understood to be
an example of internal electrophilic substitution (IES), in
agreement with similar work by Oxgaard, Periana, Goddard et
al.31 This is similar in some respects to proposed C-H activation
via electrophilic substitution by late transition metal complexes
(with the exception that the proton transfer is generally
considered an intermolecular reaction).9
We note that unlike previous computational studies15b,c of
(Tab)Ru(PH3)OH, an η2-C,C-bound benzene adduct was not
isolated in the present research, which may be a reflection of a
slight downsizing of the Ru basis set (CSDZ* is valence doubleζ; previous simulations employed the full triple-ζ contraction
for the Stevens’ valence basis sets35c) or, more likely, the
inherent weakness of the metal-benzene interaction in neutral
complexes of this type. In a previous study of benzene C-H
activation by full TpRu(L)R complexes (L ) CO, PR3 or CNH;
R ) alkyl or aryl),43 it was observed that while benzene binding
was weakly exothermic, an unfavorable entropic contribution
rendered this event endergonic. Thus, consistent with the current
calculation, the previous calculations of TpRu-methyl complexes
suggests relatively weak benzene coordination. In addition, the
(42) Even for cationic complexes, benzene adducts C are not as strongly bound
as, for example, a quintessential EAS intermediate such as protonated
benzene (B3LYP/6-311++G(d,p) geometry optimization), which has a very
long “Bz C-C” (1.47 Å) and large “Bz C-H oop” (30°). See Table 1 for
a discussion of these metrics, and comparable [M-X]q+ values.
(43) Foley, N. A.; Lail, M.; Lee, J. P.; Gunnoe, T. B.; Cundari, T. R.; Petersen,
J. L. J. Am. Chem. Soc. 2007, 129, 6765.
13176 J. AM. CHEM. SOC.
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VOL. 129, NO. 43, 2007
Figure 2. Benzene adducts of [(Tab)Co(PH3)(benzene)NH2]+ (left) and
[(Tab)Ni(PH3)(benzene)NH2]2+ (right). The Tab ligand is shown in wireframe for clarity. Pertinent metric data given in Table 1.
previous studies revealed that the details of the metal-benzene
interaction are subtle. For example, while benzene bonding
modes in TpRu(Ph)(CO)(benzene) and TpRu(Ph)(CNH)(benzene) were calculated to η2-C,C, the related complex TpRu(Ph)(PMe3)(benzene) showed η2-C,H bonding of the benzene,
which is quite similar to the adducts calculated herein, due to
steric reasons.43 Thus, given the previous subtlety of benzene
coordination mode for TpRu(L)R systems, it is not surprising
that coordination of benzene by [(Tab)M(PH3)X]q is highly
dependent on M and X.
Table 1 presents pertinent geometric data for calculated
benzene adducts. Several structural trends as a function of metal
and activating ligand are observed. Strongly bound benzene
adducts only occur for cationic complexes, e.g., [Ni-OH]2+,
[Pt-OH]2+, and [Ir-OH]+ among the hydroxo complexes, and
[Pt-NH2]2+ among the amido complexes. As expected for
electrophilic addition to benzene, dicationic group 10 metal
complexes show structural evidence of stronger benzene binding
as compared to cationic group 9 species (e.g., compare benzene
adducts of [Ir-OH]+ and [Pt-OH]2+, Table 1). The structural
assessments are made primarily on the basis of the long metalcarbon(benzene) bond distances found in the congeners containing earlier (neutral or anionic) transition metal complexes, which
substantially shortens for the later (cationic) complexes. Other
structural indicators of greater benzene/complex interaction and
activation for cationic complexes include longer C-C bond
lengths for the benzene bond closest to the metal, and pyramidalization of the carbon in the proximal C-H bond (Figure 1).
Such changes as a function of overall charge are expected and
partly reflect the lack of solvent effects in the modeling. What
is perhaps more unexpected and interesting is the role of metal
(M) for a given molecular charge. Benzene binding is stronger
for heavier congener within a triad (e.g. compare benzene
adducts of [Co-OH]+ vs [Ir-OH]+ or [Ni-NH2]2+ vs [PtNH2]2+, Table 1).
The degree of metal-NH2 π-interaction in the amido
complexes, as well as its response to modification of the
complex, is interesting and can be assessed by the “flap” angle
of the amido plane (i.e., the H‚‚‚H-N-M improper dihedral).
When the metal-benzene interaction is very weak, the amido
flap angle is close to 180°, as it is for Tc-, Re-, Ru-, Co-,
and Ir-amido complexes (179° for each). Figure 2 shows the
calculated geometry of species C for [Co-NH2]+, for which
the benzene and Co essentially do not interact. The structural
results for some amido complexes thus imply that there is
Key Step in Hydrocarbon Functionalization
ARTICLES
Table 2. Electronic Properties of 16-Electron [(Tab)M(PH3)(X)]a
LUKSOb energy (au)
NPA charge on metal (e-)
metal
OH
NH2
OH
NH2
Tc
Re
Ru
Co
Ir
Ni
Pt
0.090
0.086
-0.082
-0.283
-0.269
-0.502
-0.477
0.098
0.095
-0.072
-0.268
-0.253
-0.485
-0.457
0.153
0.277
0.330
0.705
0.690
0.863
0.907
0.062
0.195
0.233
0.628
0.597
0.831
0.851
a
Lowest unoccupied Kohn-Sham orbital (LUKSO) energies in atomic
units are determined at the calculated minima of [TabM(PH3)(X)] using
the B3LYP/CSDZ* level of theory. NPA ) natural population analysis.
b For comparison purposes the highest occupied Kohn-Sham orbital
(HOKSO) energy of benzene at the same level of theory is -0.246 au.
significant π-donation from the amido ligand to the metal center
that competes with the coordination of benzene. Alternatively,
the enhanced amido-to-metal π-bonding compensates for the
lack of metal-benzene bonding. The amido flap angle in those
cases in which the amido is not planar is reduced toward the
109.5° value expected for σ-only bonding for [Ni-NH2]2+,
Figure 2, and [Pt-NH2]2+ (123° and 116°, respectively),
implying that coordination of the benzene effectively saturates
the metal, thus ameliorating the degree of metal-amido dπpπ interaction. The foregoing comments must be tempered to
some degree as planarity of a metal-amido ligand may also
arise in the limit of a significantly ionic (LnM+NH2-) bonding
description.44
Investigation of the electronic structure of benzene adducts
(C in Scheme 4) reveals an increasing interaction between the
metal and benzene as the acidity of the metal increases. The
acidity of 16-electron complex B was assessed using the
LUKSO (lowest unoccupied Kohn-Sham orbital45) energy and
NPA charge on the metal. A lower LUKSO energy and a more
positive NPA charge imply a more acidic metal center. Since
NH2- is a stronger base than OH- (based on gas-phase proton
affinities46), we expect the metal center to be less acidic for the
amido complexes as compared to the hydroxo complexes, and
indeed this is the case. For each hydroxo species B, the
corresponding amido species is calculated to be less acidic as
indicated by the LUKSO energies and NPA charge on the metal,
Table 2.
For both the hydroxo and amido systems containing Tc, Re,
and Ru, no benzene adducts were found via DFT geometry
optimization. The HOKSO (highest occupied Kohn-Sham
orbital) energy of benzene at the level of theory used is -0.246
a.u. Analysis of LUKSO energies in Table 2 indicates that
benzene binds only in the limit that LUKSO([M-X]q) < HOKSO(benzene), supporting the conclusion made above as to the
correlation between the acidity of B and its ability to form a
stable adduct C. Caution is needed, however, in interpretation
of the present results, as experiments are conducted in condensed
media. The overall charge of the complex is an obvious factor
in regulating the interaction between the active species B and
benzene. In the absence of solvation, charge effects will
(44) Grotjahn, D. B.; Sheridan, P. M.; Jihad, I. A.; Ziurys, L. M. J. Am. Chem.
Soc. 2001, 123, 5489.
(45) Kohn-Sham orbitals, e.g., frontier orbitals HOKSO and LUKSO, are the
DFT equivalent of molecular orbitals and may be similarly interpreted.
(46) Linstrom, P. J., Mallard, W. G. Eds. NIST Chemistry WebBook, NIST
Standard Reference Database Number 69; National Institute of Standards
and Technology: Gaithersburg MD, 2005; 20899 (http://webbook.nist.gov).
Figure 3. Calculated transition state for C-H activation of benzene by
[(Tab)Pt(PH3)OH]2+. Tab ligand is shown in wireframe.
dominate. However, the comparison of NPA and LUKSO data
in Table 2 for systems with equivalent charge make it apparent
that both M and X can modify the acidity/basicity of B and
thus provide evidence for the important electronic role that the
metal M and the ligand X play in modifying the overall activity
toward C-H bond cleavage.
3.1.2. Transition States for Benzene C-H Activation. As
expected from previous studies by our group and others,13,15,36
the transition states (TSs) for C-H bond activation of benzene
by the M-X bond of [(Tab)M(PH3)(benzene)X]q have a fourcentered arrangement with a kite-shaped geometry resulting from
the obtuse angle about the hydrogen being transferred. The
imaginary frequencies in these transition states correspond
almost exclusively to C···H and X···H bond-forming/-breaking
via motion of the active hydrogen. The TS structure results in
a relatively short metal-hydrogen distance since the M···C and
M···X distances in such transition states are typically only
marginally longer (∼10-20%) than normal covalent bond
lengths. A representative example of the calculated transition
state for benzene C-H activation by [(Tab)Pt(PH3)OH]2+ is
shown in Figure 3. In these transition states the benzene ring is
more or less perpendicular to the plane defined by the X···M··
·C···H ring.
Short M···H distances in SBM transition states have been used
in the past as a gauge of metal-hydrogen interaction.47 For the
systems studied herein, the “reduced” M-H distances (calculated by subtracting the sum of the covalent radii48 of the metal
and hydrogen from the M‚‚‚H distance in transition state Dq)
were calculated. A plot of this metric is given in Figure 4,
showing the metric as a function of metal and ligand X. In
relative terms, the M···H “bond” in the transition states are on
average 8% longer than covalent estimates in the hydroxo
systems and 12% longer in the amido complexes, which could
be taken as an indicator of significant bonding between the metal
and the hydrogen of the C-H bond being activated. However,
a metric analysis may be too simplistic as there must also be
electron density along the internuclear axis.
To analyze the nature of the bonding in the transition state,
Atoms in Molecules37 (AIM) analyses were performed for
representative [Ru-OH] and [Ru-NH2] benzene C-H activation transition states. The AIM technique is a topological
analysis of the calculated electronic structure of a molecule,
which, among other procedures, searches for critical points
(stationary points) in the total electron density. Such critical
points are taken to indicate that bonding exists among two or
(47) See, for example: Cundari, T. R.; Gordon, M. S. J. Am. Chem. Soc. 1993,
115, 4210.
(48) WebElements Scholar, http://www.webelements.com/, accessed January
2007.
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ARTICLES
Scheme 6. Calculated Metric Data for C-H Activation of Benzene
by TabRu(PH3)(X), where X ) CH3,49 NH2, or OH
Table 3. Early vs Late CH Activation Transition Statesa
X ) OH
metal
D(C‚‚‚H)
(%)
D(O‚‚‚H)
(%)
Tc
Re
Ru
Co
Ir
Ni
Pt
27
24
22
19
22
15
15
10
13
15
18
18
22
25
X ) NH2
TS type
D(C‚‚‚H)
(%)
∆(N‚‚‚H)
(%)
TS type
late
late
late
middle
middle
early
early
23
21
19
14
13
12
11
19
21
23
28
32
31
36
late
middle
middle
early
early
early
early
a ∆(C‚‚‚H) or ∆(X‚‚‚H) is the calculated percent lengthening of the
particular bond in the transition state versus typical covalent single bond
lengths.
Figure 4. Reduced M-H bond distance (i.e., M···H distance in transition
state minus sum of covalent radii of metal and hydrogen) by row and group
and by X group.
Scheme 5. Two Possible Depictions from Atoms in Molecules
(AIM) Analyses for Benzene CH Activation Transition Statesa
a Square indicates a ring critical point; a circle indicates a bond critical
point. The AIM analysis is more consistent with the depiction on the right
for [Ru-OH] and [Ru-NH2] transition states.
more atoms. The AIM analysis indicates four bond critical points
(M···X, X···H, C···H, and M···C), thus implying a M···X···H···C fourmembered ring, which is confirmed by the identification of a
ring critical point in the electron density within the proposed
cycle. No bond critical point could be located between the X·
··C pairs. What is more intriguing is that the AIM analysis does
not indicate a bond critical point between the metal and
hydrogen in the active site of the transition state. Hence, the
bond connectivity in Scheme 5 (right) is suggested for these
transition states. In the limit of a M···H bond, one would also
expect two ring critical points for the individual three-membered
cycles (Scheme 5, left).
The OHM description is thus contraindicated for these
transition states on the basis of the lack of significant M-H
interaction as determined by the AIM analysis. In considering
the nature of the transition state for C-H activation, a useful
comparison can be made between the SBM transition state
indicated by the AIM analysis and IES (see above) mechanism.
The major difference between SBM and IES is the participation
of the lone pair on the ligand “X” in the C-H activation step.
13178 J. AM. CHEM. SOC.
9
VOL. 129, NO. 43, 2007
We have previously suggested the role of the lone pair for
activation of dihydrogen and intramolecular C-H activation by
(PCP)Ru(CO)NH2.29 Scheme 6 provides a comparison of the
calculated distances for the four atoms that compose the active
site for benzene C-H activation by (Tab)Ru(PH3)(C6H6)X for
X ) CH3,15c,49 NH2, and OH. It is readily apparent from the
metric data that the X groups with a lone pair (NH2 and OH)
are quite similar to each other, while the methyl-activating ligand
is more disparate. Thus, the metric data for the calculated
transition states indicate a substantial difference between X )
Me and X ) OH/NH2. The origin of this difference is likely
the presence of a lone pair on the amido and hydroxo ligands.
What is particularly noticeable is the much shorter Ru···H
distance in the transition state for X ) CH3. We propose that
the directed sp3 hybrid of the methyl-activating ligand less
effectively “bridges” both the transfer hydrogen and the metal,
and thus must compromise binding with each of these moieties.
As a result, the Ru-X-H angle is small and the resulting Ru·
··H distance short. Bercaw et al. presented a similar analysis
for the preference of H over alkyl for the transfer group in their
classic study of SBM by scandium-alkyl complexes.14 For
complexes with amido- and hydroxo-activating ligands, the
presence of available lone pairs makes these ligands more
effective at “bridging” M and H, and thus the Ru-X-H angle
can expand, and Ru···H can be longer, as depicted in Scheme 6.
As a measure of the relative position of the transition state
on the reaction coordinate (i.e., early versus late transition
states), the percent deviations from a typical single C-H and
X-H bond length (as determined by the sum of covalent radii)
for both hydroxo and amido ligands are presented in Table 3.
Inspection of the data in Table 3 indicates that the transition
(49) Lail, M.; Bell, C. M.; Cundari, T. R.; Conner, D.; Gunnoe, T. B.; Petersen,
J. L. Organometallics 2004, 23, 5007.
Key Step in Hydrocarbon Functionalization
states are late for structures with electron-rich (anionic) metal
centers (e.g., [Tc-OH]-, [Re-OH]-, and [Tc-NH2]-) and
become progressively earlier for more acidic (cationic) metal
centers. The calculations are consistent with more electron-rich
metal centers being more efficient at promoting the donation
of electron density into the C-H σ* orbital, causing a longer
(later) C-H bond length in the transition state.12 The amido
transition states are relatively early as compared to those of their
hydroxo congeners, Table 3, which is expected of the more basic
amido ligand under the IES mechanism.
One might ask why oxidative hydrogen migration was
observed as the mechanism in similar systems14b,15,49,50 but not
here. The ligand receiving the transferred hydrogen is an alkyl
group in systems showing OHM as a mechanism (e.g., (Tab)Ru(L)(C6H6)R), but the current research employs heteroatomic
hydroxo and amido as the activating ligands. The most obvious
difference between these X groups is the existence of lone pairs
on the hydroxo and amido ligands that are missing in the alkyl
systems. Indeed, an AIM analysis of the X ) CH3 transition
state lacks a four-membered ring critical point and instead
indicates true Ru-H(ipso) interaction as prescribed by the OHM
mechanism. It is reasonable that rather than a metal dπ orbital,12
perhaps an available lone pair on X is responsible for the
donation of electron density to the activated hydrogen.12 That
is, when the ligand X possesses a lone pair, it functions as an
intramolecular base, which is consistent with the IES mechanism
and highlights a fundamental difference between heteroatomand hydrocarbyl-activating ligands. In fact, on the basis of this
premise, we suggest that SBM/IES type C-H activation might
be inherently more facile when the “receiving ligand” is anionic
and heteroatomic than for hydrocarbyl ligands. Future efforts
will address this issue in more detail. To assess the role of the
X group using metric data, the M-X bond lengths from the
active species (B), the transition states (Dq), and products (E)
are compared. If the donation to σ*CH is primarily from the
ligand, the M-X bond length in the transition state is expected
to more closely resemble the product. Conversely, if the donation
to σ*CH is primarily from the metal center (i.e., substantial M-H
interaction), we propose that the transition state M-X bond
length should be no greater than halfway between the corresponding values calculated for B and E. In all cases, the M-X
bond lengths in the transition states are closest to the product
values (Table 4) on average 64% for hydroxo and 70% for
amido, a structural indicator of a more significant contribution
to C-H bond scission from the ligand than the metal.
These results suggest that the mechanism for C-H activation
by [(Tab)M(PH3)X]n fragments (where X ) a formally anionic
heteroatomic ligand such as hydroxo or amido) is perhaps best
viewed as an intramolecular proton transfer or IES as defined
by Goddard, Periana, et al. (Scheme 7).31 An important
mechanistic distinction between IES and an aromatic substitution
is based on the interaction of the metal with the C-H bond
rather than the arene π-system. Upon coordination to the metal
center, acidic character is imparted to the C-H bond, resulting
in relatively facile proton transfer to the basic ligand “X.”
Calculated results for the transition state of benzene C-H
activation that are consistent with the intramolecular protontransfer view include: (1) shorter Ru-H bond distance for X
(50) (a) Oxgaard, J.; Periana, R. A.; Goddard, W. A., III. J. Am. Chem. Soc.
2004, 126, 11658. (b) Oxgaard, J.; Goddard, W. A., III. J. Am. Chem. Soc.
2004, 126, 442.
ARTICLES
Table 4. Comparison of M-X Bond Lengths (Å) between the
Active Species, Transition State, and Producta
metal
X
active
species
(B)
Tc
Re
Ru
Co
Ir
Ni
Pt
Tc
Re
Ru
Co
Ir
Ni
Pt
OH
OH
OH
OH
OH
OH
OH
NH2
NH2
NH2
NH2
NH2
NH2
NH2
2.03
2.00
1.97
1.77
1.92
1.76
1.92
2.01
1.99
1.95
1.77
1.95
1.77
1.91
transition
state
(Dq)
product
(E)
percent
completion
2.30
2.28
2.20
1.94
2.15
1.92
2.11
2.23
2.21
2.17
1.95
2.12
1.95
2.11
2.44
2.40
2.30
2.06
2.23
2.07
2.25
2.31
2.29
2.25
2.05
2.19
2.05
2.20
66
68
69
60
75
53
58
75
75
74
66
70
63
69
a Percent completion is the ratio of the transition-state bond length to
the product bond length, relative to the active species’ bond length, e.g.,
50% completion means the transition-state bond length is exactly halfway
between the active species and product bond lengths.
Scheme 7. Proposed Mechanism for C-H Activation by
[(Tab)M(PH3)(C6H6)X]n (X ) OH or NH2) Is Best Described as an
Intramolecular Proton Transfer
) Me than for X ) OH or NH2, (2) shorter M-H bond
distances for less basic OH ligands than for NH2 ligands, and
(3) longer M-H bond distances for later and more electrophilic
metals (Figure 4). Such a reaction is similar to net heterolytic
cleavage of dihydrogen by transition metal-amido complexes
and has been previously discussed for intramolecular C-H
activation by a Ru(II) parent amido system.25,29,51 In addition,
related mechanistic issues and conclusions have been discussed
recently in an excellent detailed computational analysis including
an interesting explicit investigation of orbital transformations
involved in C-H activation by an Ir(III)-methoxo complex.31
3.2. Kinetic and Thermodynamic Considerations. Analysis
of the structural and electronic properties of the various
stationary points along the reaction pathway for 1,2-addition
of the C-H bond of benzene has revealed a considerable degree
of sensitivity to modification of the metal, including overall
complex charge and the activating ligand X. It is also likely
that exchange of the phosphine ligand (PH3 for the calculations)
also influences the energetics of C-H activation.43 From the
perspective of rational design of activating complexes, this
flexibility is highly desirable as it suggests considerable ability
to tune d6 TpM(L)X complexes toward optimal activity and
selectivity. Attention is now turned to issues of kinetics and
(51) (a) Sandoval, C. A.; Ohkuma, T.; Muniz, K.; Noyori, R. J. Am. Chem.
Soc. 2003, 125, 13490. (b) Murata, K.; Konishi, H.; Ito, M.; Ikariya, T.;
Organometallics 2002, 21, 253. (c) Guo, R.; Morris, R. H.; Song, D. J.
Am. Chem. Soc. 2005, 127, 516. (d) Clapham, S. E.; Hadzovic, A.; Morris,
R. H. Coord. Chem. ReV. 2004, 248, 2201. (e) Abdur-Rashid, K.; Clapham,
S. E.; Hadzovic, A.; Harvey, J. N.; Lough, A. J.; Morris, R. H. J. Am.
Chem. Soc. 2002, 124, 15104. (f) Abdur-Rashid, K.; Faatz, M.; Lough, A.
J.; Morris, R. H. J. Am. Chem. Soc. 2001, 123, 7473. (g) Heiden, Z. M.;
Rauchfuss, T. B. J. Am. Chem. Soc. 2006, 128, 13048. (h) Fryzuk, M. D.;
Montgomery, C. D.; Rettig, S. J. Organometallics 1991, 10, 467.
J. AM. CHEM. SOC.
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ARTICLES
Table 5. Free Energies (kcal/mol) for Generation of 16-Electron
Active Species [(Tab)M(PH3)X]q (B) from [(Tab)M(PH3)2X]q (A)a
M
X ) OH
X ) NH2
M
X ) OH
X ) NH2
Tc
Re
Ru
Co
-4.2
-2.2
-0.9
7.8
-5.7
-5.0
-4.6
-1.8
Ir
Ni
Pt
13.6
18.7
27.6
2.8
10.6
14.9
a This is the B3LYP/CSDZ* calculated free energy for the reaction,
TabM(X)(PH3)2 f TabM(X)(PH3) + PH3.
Table 6. Free Energies (kcal/mol) of Activation of Benzene C-H
Bond Starting from (Tab)M(PH3)2X (A)a
M
X ) OH
X ) NH2
M
X ) OH
X ) NH2
Tc
Re
Ru
Co
26.5
33.0
29.6
28.7
27.0
32.4
29.1
25.5
Ir
Ni
Pt
35.2
19.4
24.1
32.0
18.9
22.4
a This is the B3LYP/CSDZ* calculated free energy difference between
transition state Dq and reactants A.
Figure 5. Calculated ∆G for benzene C-H activation by the complexes
(Tab)M(PH3)2X for X ) OH (top) and X ) NH2 (bottom). D* represents
the transition state for C-H activation.
thermodynamics to probe whether the calculated molecular and
electronic structural changes discussed above are manifested
in the energetics of C-H bond activation of benzene by the
model scorpionate complexes.
An overview of the proposed reaction pathway for benzene
C-H activation is given in Scheme 4. The overall transformation involves the loss of phosphine from the 18-electron
precursor [(Tab)M(PH3)2X]q (A) to generate the formally 16electron species [(Tab)M(PH3)X]q (B). Benzene then coordinates
to B to form the adduct [(Tab)M(PH3)(benzene)X]q (C), which
is followed by the transition state for benzene C-H activation
(Dq) leading to 18-electron product (Tab)M(HX)(PH3)(Ph) (E).
The dative ligand XH may then be replaced by the original PH3
to yield bis-phosphine complex (Tab)M(PH3)2(Ph) (F).15b,c
Calculated free energies for these steps are depicted in Figure
5 for [M-OH]q (top) and [M-NH2]q (bottom) complexes.
Three thermodynamic criteria were used to assess the
suitability of different metal-ligand combinations for C-H
bond activation of benzene. The first criterion we evaluated is
the energetic barrier to the formation of the unsaturated species
B from the precursor A, as C-H activation of benzene requires
coordination to the metal center. Second, the magnitude of the
benzene C-H activation transition state Dq relative to starting
materials A is considered. The third criterion assessed here is
the thermoneutrality of the proton-transfer step (B + PhH f
E), which is related to incorporation of the C-H activation
sequence into potential catalytic cycles. For efficient catalysis,
this reaction should be close to thermoneutral in order to avoid
thermodynamic “sinks.”
3.2.1. Generation of 16-Electron Active Species. The
generation of the active species B plays an obviously important
role in determining the overall rate of benzene C-H activation.
If the unsaturated complex is too high in energy, activation of
the C-H bond will be hampered by a low concentration of
active species B or, alternatively, require a larger concentration
13180 J. AM. CHEM. SOC.
9
VOL. 129, NO. 43, 2007
of the precursor complex A. The free energies to create the 16electron active species are given in Table 5. In general, the
extrusion of phosphine becomes more endergonic as the charge
on the complex becomes more positive, which is consistent with
tighter binding of the Lewis basic phosphine to a more acidic
metal center. However, given the favorable entropy for a bond
dissociation process, the overall ∆Gbind(PH3) for this step is not
inordinate for any complex, being less than 28 kcal/mol for even
the most tightly bound system, [Pt-OH]2+, and much less than
this for all other complexes studied.
Phosphine loss is more favorable for the amido complexes
than for their hydroxo counterparts, suggesting an advantage
for the former in the way of a greater concentration of active
species for [M-NH2] than for the corresponding [M-OH]
complexes. We hypothesize that this difference is a consequence
of greater π-donation for the amido ligands versus hydroxo
ligands, as discussed above, which nominally puts the active
species closer to a more stable 18-electron count. Hence, from
the perspective of generation of 16-electron active species B,
amido complexes are expected to have an advantage over their
hydroxo counterparts.52
3.2.2. Kinetic Barrier to C-H Bond Activation of Benzene.
In terms of targeting new systems capable of facile C-H
activation via 1,2-addition across M-X bonds, the most
pertinent energetic parameter is the activation barrier for the
C-H bond-breaking event. For convenience and ease of
comparison, we have defined this barrier as the calculated free
energy difference between the precursor complex (Tab)M(PH3)2X (A) and the transition states (Dq, Table 6).
The transition states for benzene C-H activation, Dq, fall
within the free energy range of ∼19 kcal/mol ([Ni-X]2+ for X
) OH, NH2) to ∼35 kcal/mol ([Ir-OH]+) above precursors A.
For systems with the same total charge, progressing from a first
or second row transition metal to the third row is calculated to
increase the barrier to benzene C-H activation. For example,
(52) In some cases, the loss of phosphine is calculated to have a slightly favorable
change in Gibbs free energy due to entropic factors. Since these are gasphase calculations, the effects of entropy in the dissociation step are
magnified beyond what is expected in solvent. However, the contribution
of T∆S is roughly constant (within 1.8 kcal/mol) across all metals for this
step. Further, the entropic contribution to the AfB and BfC steps roughly
cancel each other (within 4.5 kcal/mol), and the remaining steps are
unimolecular, effectively eliminating solvation concerns for the purposes
of this study.
Key Step in Hydrocarbon Functionalization
ARTICLES
Table 7. Free Energies (kcal/mol) of Hydrogen Transfera
M
X ) OH
X ) NH2
M
X ) OH
X ) NH2
Tc
Re
Ru
Co
24.5
26.4
16.2
1.9
12.7
15.6
4.4
-6.4
Ir
Ni
Pt
-1.7
-27.4
-31.2
-9.6
-31.2
-33.3
a This is the B3LYP/CSDZ* calculated free energy difference between
active species B plus benzene and 18-electron product E, TabM(HX)(PH3)(Ph).
the free energies of activation for the heavier rhenium complexes
are ∼6 kcal/mol higher than those of the lighter technetium
congeners. Likewise, there is an increase in free energy of
activation for Ir as compared to that for Co by 6.5 kcal/mol
(for both X ) OH and NH2) and also platinum versus nickel
(difference of 4.7 kcal/mol for X ) OH and 3.5 kcal/mol for X
) NH2).
With one exception (Tc), the calculated barriers for the amido
complexes are lower than those for their hydroxo counterparts
by an average of 1.6 kcal/mol for the remaining six systems.
The largest calculated difference is 3.2 kcal/mol for the Co and
Ir systems, while the smallest magnitude difference is 0.5 kcal/
mol for Tc and Ni. The magnitude of the free energy change
for A f Dq is smaller for variation of X than the change due
to variation of metal (same ligand X) within a triad. Computational studies of 1,2-addition of a C-H bond of methane across
the Ir-X (X ) OH, OMe, OCF3, or NH2) bond of the model
complexes Ir(Me)2(NH3)2(X)(CH4) reveal a difference in ∆Hq
that is less than 2 kcal/mol for variation of X, which is similar
in magnitude to our calculated differences upon variation of X
for (Tab)M complexes.31 However, our comparison of activation
barriers for hydroxo versus those for amido are for oVerall
reactions that involve phosphine dissociation, benzene coordination, and the C-H activation step. For a more direct comparison
to the results for Ir(Me)2(NH3)2(X)(CH4) systems, we compared
the ∆∆Gq for 1,2-addition of the C-H bond of benzene starting
from [(Tab)Ir(PH3)(C6H6)X]+complexes. The ∆Gq for the 1,2addition is 17.6 kcal/mol for X ) OH and 28.9 kcal/mol for X
) NH2. Thus, we calculate a much more substantial change in
activation barrier upon substitution of OH with NH2 for (Tab)Ir(PH3)(C6H6)X systems with ∆∆Gq ) 11.3 kcal/mol [compared
to e2 kcal/mol difference for Ir(Me)2(NH3)2(X)(CH4)]. In part,
these differences may reflect the impact of gas-phase calculations that incorporate entropy (this study) versus changes in
enthalpy (previous study of Ir system31). Combined with the
structural eVidence discussed aboVe, the calculations yield an
emerging picture that catalysis will be significantly influenced
by both the metal M and the actiVating ligand X.
3.2.3. Thermoneutrality of Hydrogen Transfer. For the
possible incorporation of the C-H activation sequences into
catalytic cycles, the overall thermodynamics of the hydrogentransfer step (i.e., ∆Gtrans, the free energy of the reaction (Tab)M(PH3)X [B] +PhH f (Tab)M(PH3)(HX)Ph [E]) are important.
Effective catalysis implies that these transformations be neither
too favorable nor too unfavorable, as indicated in previous
theory-experiment investigations of the observed efficient H/D
exchange.15c The Ni, Pt, and Ir complexes for both activating
ligands X and the Co-amido complex are exergonic for the
conversion of B plus benzene to E (Table 7), implying these
metals may be prime candidates for direct observation of
benzene C-H activation and functionalization. Consistent with
our calculations, Periana et al. have observed the conversion of
an Ir(III)-methoxo complex and benzene to methanol and the
corresponding Ir(III)-phenyl system.15a However, for the Ni
and Pt model systems, the benzene C-H activations are
calculated to be highly exergonic, implying that these systems
may be less suitable for incorporation into catalytic sequences.
The systems for which the B f E transformation are calculated
to be closest to thermoneutral are the [Co-OH]+ and [Ir- OH]+
complexes.
For a given metal, the amido ligand lowers ∆G for conversion
of [(Tab)M(PH3)X]q (B) plus benzene to [(Tab)M(PH3)(XH)Ph]q (E), the free energy change for the hydrogen-transfer step,
in comparison to the hydroxo ligand. The differences in Gibbs
free energy range from 12.8 kcal/mol for Tc to 2.1 kcal/mol
for Pt. In general, the difference between hydroxo and amido
decreases from left to right in the transition series.53
4. Summary, Conclusion, and Prospectus
The feasibility and kinetic accessibility of the C-H activation
step in a catalytic cycle like that shown on the right side of
Scheme 2 have been demonstrated (at least for benzene
activation) using Ru and Ir complexes.13,15 Although much
remains to be learned about these transformations, it can now
be said that such reactions are accessible with late transition
metal systems and that the activation barriers for C-H activation
are reasonably low for implementation into catalytic cycles. The
second key step, net oxygen atom insertion into an M-R or
M-Ar bond, is at least equally challenging. Despite the utility
and interest in these reactions, insertion of oxygen into metalalkyl or -aryl functionalities has rarely been directly observed.20,21 Mayer and Brown have reported the conversion of
Re(VII)-oxo complexes with phenyl ligands to the corresponding phenoxo complexes, and in at least one case the transformation occurs under thermal conditions.20 More recently, Periana
et al. have reported the reaction of methylrheniumtrioxo with
external oxidants (e.g., hydrogen peroxide, pyridine-N-oxide,
periodate, and iodosyl benzene) to form Re(OMe)O3.21 Importantly, these transformations occur relatively rapidly at room
temperature, and preliminary mechanistic studies indicate that
the oxygen atom in the final methoxo ligand does not originate
from a Re-oxo moiety. A mechanism similar to the classic
Baeyer-Villiger organic reaction54 has been proposed.
Thus, the two key steps in the catalytic cycle depicted on
the right side of Scheme 2 have been observed; however, the
C-H activations have been observed for systems with high
d-electron counts [i.e., d6 for Ru(II) and Ir(III)],13,15 while the
transformation of M-R into M-OR have only been observed
for systems in high oxidation states (and thus low d-electron
counts).20,21 For the present models we have kept the d-electron
count fixed at six, which has necessitated variation of the overall
charge. While the precise relationship between the kinetics and
thermodynamics of these transformations Vis-à-Vis d-electron
counts and metal formal oxidation states is not entirely
understood, it is reasonable to expect that a marriage of these
two transformations may require a catalyst that satisfies both
requirements (such as CoIII, PtIV, etc.).
(53) For example, the average difference is 11.8 kcal/mol for group 7, 11.8
kcal/mol for group 8, 8.1 kcal/mol for group 9, and 2.9 kcal/mol for group
10.
(54) Brink, G. J.; Arends, I. W. C. E.; Sheldon, R. A. Chem. ReV. 2004, 104,
4105.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 43, 2007 13181
Cundari et al.
ARTICLES
Tables 5-7 provide a comparison of the three criteria
considered to quantify the efficacy of (Tab)M complexes toward
C-H activation and their possible use as catalysts (i.e.,
generation of the active species, overall hydrogen-transfer
barrier, and thermoneutrality of the hydrogen-transfer step) for
hydrocarbon functionalization. As expected, generation of the
coordinatively unsaturated species [(Tab)M(PH3)X]q (B) via loss
of phosphine from the 18-electron precursors A becomes less
favorable as the effective charge on the metal complex becomes
more positive. As the charge on the metal complex becomes
more positive, ∆Gq (A + PhH f Dq) and ∆G (B + PhH f E)
for C-H activation step are both reduced (i.e., become more
favorable). However, the present calculations suggest that the
metal plays a primary role in the kinetic and thermodynamic
feasibility of arene C-H bond activation and functionalization.
In addition to the identity of the metal, the ligand X impacts
the calculated reaction energetics.55 Pt is the metal most
“tunable” by the ligand X, with a mean absolute change of 11.5
kcal/mol in ∆G due to change of ligand “X.” On average, the
generation of the 16-electron active species B from A by
phosphine loss is ∼7 kcal/mol more facile for the amido than
their hydroxo congeners, Table 5. Likewise, the A + PhH f
Dq activation barriers are ∼1-3 kcal/mol lower for X ) NH2
than X ) OH. Both of these observations suggest an advantage
for amido over hydroxo complexes. It is interesting to note that
the energetic discrepancy between hydroxo and amido complexes becomes greater for these two criteria as one moves from
left to right in the transition series. For the thermoneutrality (of
hydrogen transfer) criterion, hydroxo and amido complexes are
comparable in a global sense, with the early metals being closer
to thermoneutral for the amido than the hydroxo complexes and
vice versa for the later metal models.
Here it should be noted that, while the C-H activation step
is important and is the RDS for most calculated systems, the
energetics of PH3/benzene exchange is also a potentially
significant contributor to the success of C-H activation. This
underscores the point that hydrocarbon coordination is often as
challenging as the actual C-H bond cleavage step. Thus, when
considering the impact of variation of the ligand “X,” the
influence on hydrocarbon coordination should also be considered, especially when varying “X” from alkyl or aryl to π-donor
heteroatomic ligands such as OR or NHR, which can impact
ligand coordination dramatically. For example, the complexes
TpRu(PMe3)2X (X ) OPh, OH or NHPh) exhibit more rapid
rates of dissociative phosphine exchange compared to TpRu(PMe3)2R (R ) Ph or Me).15c
A simple linear catalyst scoring function can be constructed
using the calculated ∆G values for the three criteria enumerated
above (see Tables 5-7) to suggest a good balance between
competing catalytic trends.56 Assigning a lower priority to the
free energy of formation for the active species (A f B)57
indicates [Co-OH]+, [Ir-OH]+, [Ru-NH2], [Co-NH2]+, and
[Ir-NH2]+ are the best candidates for further experimental
study, i.e., they have the lowest catalyst scores. It is worth noting
(55) For each step, the average range of free energy values is ca. 20 kcal/mol
(holding the ligand constant, for both ligands), whereas the range of the
difference between the two ligands for each step (i.e., holding the metal
constant) is ca. 13 kcal/mol, only 65% of the variation due to the metal.
(56) The weighted sum of the ∆G values of the corresponding criteria may be
combined to produce a score, S ) wact∆Gact + wbarrier∆Gbarrier + wxfer|∆Gxfer|,
with lower scores being more desirable (see Tables 5-7 for ∆G values).
(57) Specifically, wact ) 1 and wbarrier ) wxfer ) 2.
13182 J. AM. CHEM. SOC.
9
VOL. 129, NO. 43, 2007
that the catalyst scoring function is robust in that the top
candidates did not change upon variation of the weights over a
reasonable range of values. MoreoVer, it is interesting to note
that the catalyst scoring function independently arriVes at
models of two heaVily studied and successful hydroarylation
catalysts, i.e., Ru(II) and Ir(III) complexes.13,15 These candidates
have free energies to formation of the active species less than
10 kcal/mol (except [Ir-OH]+, which has a barrier of 14 kcal/
mol), overall barriers to C-H activation (A f Dq) of less than
30 kcal/mol (except [Ru-NH2], with a value of 34 cal/mol due
to the stability of B), and are thermoneutral in the hydrogentransfer step to within 10 kcal/mol. The Co and Ir species have
the additional advantage of forming reasonable benzene adducts,
leading to enhancement in the Arrhenius prefactor for the
hydrogen-transfer step. It is also interesting to note that none
of these candidates exhibit a late transition state structure (see
Table 3). Among all the candidates, [Co-OH]+ has the most
favorable “score”56,57 and appears to be the most promising for
further research and tuning to optimize the potential catalytic
activity on the basis of an accessible barrier to hydrogen transfer
(21 kcal/mol), near thermoneutrality for hydrogen transfer (+2
kcal/mol), and small barrier to formation of the active species
(8 kcal/mol).
The AIM analysis as well as calculated structural metrics and
energy barriers implicates a fundamental shift in the nature of
the bonding in the transition state upon going from an X with
no available lone pairs (i.e., X ) methyl) to heteroatom X
groups with available lone pairs such as OH and NH2. We
propose that, while the mechanism of C-H activation for X )
hydrocarbyl is more akin to an OHM/SBM description, the
transition states for X ) OH, NH2 more closely resemble those
envisaged for Shilov-type systems in which hydrogen transfer
is an intramolecular process. In terms of development, X )
heteroatom systems may provide more profitable systems for
design and fine-tuning of hydrocarbon functional catalysts,
generally, and hydroarylation catalysts, specifically. Indeed, such
integrated theory-experiment studies are underway in our
research laboratories.
Acknowledgment. T.B.G. acknowledges the National Science
Foundation (CAREER Award; CHE 0238167) and the Alfred
P. Sloan Foundation (Research Fellowship) for financial support
of this research. T.R.C. acknowledges the U.S. Department of
Education for its support of the CASCaM facility. The research
at UNT was supported in part by a grant from the Offices of
Basic Energy Sciences, U.S. Department of Energy (Grants No.
DEFG02-03ER15387). Calculations employed the UNT computational chemistry resource, which is supported by the NSF
through Grant CHE-0342824.
Supporting Information Available: Details of calculated
stationary points; the complete citation for ref 38 (listed as ref
6 for Supporting Information refs). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA074125G